Mineral oil based lubricants are conventionally produced by a separative sequence carried out in the petroleum refinery which comprises fractionation of a paraffinic crude oil under atmospheric pressure followed by fractionation under vacuum to produce distillate fractions (neutral oils) and a residual fraction which, after deasphalting and severe solvent treatment may also be used as a lubricant basestock. This refined residual fraction is usually referred to as bright stock. Neutral oils, after solvent extraction to remove low viscosity index (V.I.) components, are conventionally subjected to dewaxing, either by solvent or catalytic dewaxing processes, to achieve the desired pour point. The dewaxed lube stock may be hydrofinished to improve stability and remove color bodies. Viscosity Index (V.I.) is a reflection of the amount of viscosity decrease a lubricant undergoes with an increase in temperature. The products of solvent dewaxing are dewaxed lube oil and slack wax. Slack wax typically contains 60% to 90% wax with the balance being entrained oil. In some instances it is desirable to purify the slack wax of entrained oil by subjecting the slack wax to a deoiling step in which the slack wax is diluted with dewaxing solvents and filtered at a temperature higher than that used in the filtering step used to produce the slack wax. The purified wax is termed deoiled wax, and contains greater than 95% wax. The byproduct of the second filtration typically contains 50% wax and is termed foots oil.
Catalytic dewaxing of lube stocks is accomplished by converting waxy molecules to light products by cracking, or by isomerizing waxy molecules to form species which remain in the dewaxed lube. Dewaxing catalysts preserve high yield primarily by having pore structures which inhibit cracking of cyclic and highly branched species, those generally associated with dewaxed lube, while permitting easier access to catalytically active sites to near-linear molecules, of which wax is generally composed. Catalysts which significantly reduce the accessibility of species on the basis of molecular size are termed shape selective. Increasing the shape selectivity of a dewaxing catalyst will frequently increase the yield of dewaxed oil.
The shape selectivity of a dewaxing catalyst is limited practically by its ability to convert waxy molecules which have a slightly branched structure. These types of species are more commonly associated with heavier lube stocks, such as bright stocks. Highly shape selective dewaxing catalysts may be unable to convert heavy, branched wax species leading to a hazy lube appearance at ambient temperature and high cloud point relative to pour point.
Conventional lube refining techniques rely upon the proper selection and use of crude stocks, usually of a paraffinic character, which produce lube fractions with desired qualities in adequate amounts. The range of permissible crude sources may, however, be extended by the lube hydrocracking process which is capable of utilizing crude stocks of marginal or poor quality, usually with a higher aromatic content than the better paraffinic crudes. The lube hydrocracking process, which is well established in the petroleum refining industry, generally comprises an initial hydrocracking step carried out under high pressure, at high temperature, and in the presence of a bifunctional catalyst which effects partial saturation and ring opening of the aromatic components which are present in the feed. The hydrocracked product is then subjected to dewaxing in order to reach the target pour point since the hydrocracked product usually contains species with relatively high pour points. Frequently the liquid product from the dewaxing step is subjected to a low temperature, high pressure hydrotreating step to reduce the aromatic content of the lube to the desired level.
Current trends in the design of automotive engines are associated with higher operating temperatures as the efficiency of the engines increases. These higher operating temperatures require successively higher quality lubricants. One of the requirements is for higher viscosity indices (V.I.) in order to reduce the effects of the higher operating temperatures on the viscosity of the engine lubricants. High V.I. values have conventionally been attained by the use of V.I. improvers e.g. polyacrylates and polystyrenes. V.I. improvers tend to undergo degradation due to high temperatures and high shear rates encountered in the engine. The more stressing conditions encountered in high efficiency engines result in even faster degradation of oils which employ significant amounts of V.I. improvers. Thus, there is a continuing need for automotive lubricants which are based on fluids of high Viscosity Index and which are resistant to the high temperature, high shear rate conditions encountered in modern engines.
Synthetic lubricants produced by the polymerization of olefins in the presence of certain catalysts have been shown to possess excellent V.I. values, but they are relatively expensive to produce. There is therefore a need for the production of high V.I. lubricants from mineral oil stocks which may be produced by techniques comparable to those presently employed in petroleum refineries.
U.S. Pat. No. 4,975,177 discloses a two-stage dewaxing process for producing lube stocks of high V.I. from waxy feedstocks. In the first stage of this process, the waxy feed is catalytically dewaxed by isomerization over zeolite beta. The product of the isomerization step still contains waxy species and requires further dewaxing to meet target pour point. The second-stage dewaxing employs either solvent dewaxing, in which case the rejected wax may be recycled to the isomerization stage to maximize yield, or catalytic dewaxing. Catalysts which may be used in the second stage are ZSM-5, ZSM-22, ZSM-23, and ZSM-35. To preserve yield and V.I., the second stage dewaxing catalyst should have selectivity similar to solvent dewaxing. U.S. Pat. No. 4,919,788 also teaches a two-stage dewaxing process in which a waxy feed is partially dewaxed by isomerization over a siliceous Y or beta catalyst with the product subsequently dewaxed to desired pour point using either solvent dewaxing or catalytic dewaxing. Dewaxing catalysts with high shape selectivity, such as ZSM-22 and ZSM-23, are preferred catalysts. These examples, however, do not teach synergistic effects involving more than one dewaxing catalyst.
Ser. No. 08/017,949 discloses a two stage hydrocracking and hydroisomerization process. The first stage employs a bifunctional catalyst comprising a metal hydrogenation component on an amorphous acidic support. The second stage, the hydroisomerization step, is carried out over zeolite beta. Subsequent dewaxing is optional but recommended. Either solvent dewaxing or catalytic dewaxing maybe used subsequently in order to obtain target V.I. and pour point. There is no teaching of catalytic synergism in this invention.
In Ser. No. 08/017,955, petroleum wax feed is subjected to hydroisomerization over a noble metal-containing zeolite catalyst of low acidity. The paraffins present in the feed are selectively converted to iso-paraffins of high V.I. but lower pour point so that a final lube product of good viscometric properties is produced with a minimal degree of subsequent dewaxing. The process, which operates under high pressure, is well suited for upgrading waxy feeds such as slack wax with aromatic contents greater than about 15 wt. % to high Viscosity Index lubricating oils with high single pass yields and limited requirement for product dewaxing.
Related cases primarily emphasize solvent dewaxing with Catalytic dewaxing as a possible alternative or secondary step. The advantage of solvent dewaxing the product of the isomerization stage in that wax is rejected and can be recycled to the isomerization catalyst to improve the yield of high V.I. lube. However, operational costs for solvent dewaxing are higher than for catalytic dewaxing. Additionally, the pour point of the solvent dewaxed lube stock is restricted by solvent refrigeration capability to approximately -5.degree. to 0.degree. F. Catalytic dewaxing enables production of high V.I. lubes having pour points significantly lower than those possible for solvent dewaxing. An unexpected development of the total catalytic dewaxing process is that it can produce lubricants with equivalent or higher V.I. at equivalent or lower pour points than lubricants produced by solvent dewaxing. U.S. Pat. No. 5,302,279 (and the analogous European patent application EP 464 547 A1) teaches the use of a low acidity form of zeolite beta for isomerizing and dewaxing furfural raffinates. The improved selectivity of a catalyst of low acidity over one of high acidity was demonstrated in the examples.